<?php

$a=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
$count = count($a);

for($i = 1; $i <= $count; $i += 2) {
  $new[] = ($a[$i]);
}
  
print_r($a);
echo PHP_EOL;
print_r($new);